Related art resin gears are configured such that an outer peripheral resin is joined to an outer circumferential surface of a metal core. For example, a worm wheel of a power steering apparatus may be configured as a resin gear in which an outer peripheral resin is joined to the outer circumferential surface of a metal core and is provided with gear teeth (see, e.g., JP2001-141033A and JP2011-106575A). By using a resin, weight of the worm wheel is reduced and a gear meshing sound is prevented.
The metal core has a boss portion into which a shaft is press-fitted, a ring portion to which an outer peripheral resin is joined, and a connecting portion connecting the boss portion and the ring portion. To the shaft, a steering force is transmitted from a steering wheel, and a reaction force is transmitted from the road surface. Therefore, sufficient joining strength is required between the boss portion and the shaft.
The metal core disclosed in JP2001-141033A is manufactured by forging a disk-shaped metal material to have a shape like a deep dish and by forming a shaft hole in the boss portion an outer circumferential concave-convex section on the ring portion. On the other hand, according to the example disclosed in JP2011-106575A, the metal core is manufactured by, among cutting, forging, sheet-metal pressing and the like, pressing a sheet metal to save cost.
According to the example disclosed in JP2001-141033A, because the metal core is manufactured by forging to have a shape like a deep dish, it is possible to reduce weight while ensuring strength by wall thickness. However, the metal core is large in the axial direction. If the boss portion and the ring portion are designed to be arranged in a radially overlapping manner to reduce the size in the axial direction, the pressing pressure needs to be significantly increased, because thick material is used to ensure sufficient strength. This consequently requires large facilities, and manufacturing is not easy.
According to the example disclosed in JP2011-106575A, the metal core is manufactured by a sheet-metal pressing. By using a thin plate, it is possible to easily arrange the boss portion and the ring portion in a radially overlapping manner so as to reduce the size and weight of the metal core. However, because the strength of metal core is lowered, the shaft cannot be press-fitted with a sufficient strength. It is possible to ensure strength by using a thicker plate. This, however, increases the weight of the metal core as well as the pressing pressure. Therefore, large facilities are required, and manufacturing is not easy.